Nanostructures and methods for manufacturing the same

ABSTRACT

A resonant tunneling diode, and other one dimensional electronic, photonic structures, and electromechanical MEMS devices, are formed as a heterostructure in a nanowhisker by forming length segments of the whisker with different materials having different band gaps.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.10/613,071, filed on Jul. 7, 2003, which claims the benefit of thepriority of U.S. Provisional Application No. 60/393,835, filed Jul. 8,2002, and of U.S. Provisional Application No. 60/459,982, filed on Apr.4, 2003, each of which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to structures, essentially inone-dimensional form, and which are of nanometer dimensions in theirwidth or diameter, and which are commonly known as nanowhiskers,nanorods, nanowires, nanotubes, etc.; for the purposes of thisspecification, such structures will be termed “one-dimensionalnanoelements”. More specifically, but not 20 exclusively, the inventionrelates to nanowhiskers, and to methods of forming nanowhiskers.

2. Brief Description of the Prior Art

The basic process of whisker formation on substrates, by the so-calledVLS (vapor-liquid-solid) mechanism is well known. A particle of acatalytic material, usually gold, for example, on a substrate is heatedin the presence of certain gases to form a melt. A pillar forms underthe melt, and the melt rises up on top of the pillar. The result is awhisker of a desired material with the solidised particle meltpositioned on top—see “Growth of Whiskers by the Vapour-Liquid-SolidMechanism”—Givargizov—Current Topics in Materials Science Vol. 1, pages79-145—North Holland Publishing Company 1978. The dimensions of suchwhiskers were in the micrometer range.

International Application WO 01/84238 discloses in FIGS. 15 and 16 amethod of forming nanowhiskers wherein nanometer sized particles from anaerosol are deposited on a substrate and these particles are used asseeds to create filaments or nanowhiskers. For the purposes of thisspecification the term nanowhiskers is intended to mean one dimensionalnanoelements with a diameter of nanometer dimensions, the element havingbeen formed by the VLS mechanism.

Typically, nanostructures are devices having at least two dimensionsless than about 1 μm (i.e., nanometer dimensions). Ordinarily, layeredstructures or stock materials having one or more layers with a thicknessless than 1 μm are not considered to be nanostructures, althoughnanostructures may be used in the preparation of such layers, as isdisclosed below. Thus the term nanostructures includes free-standing orisolated structures having two dimensions less than about 1 μm whichhave functions and utilities that are different from larger structuresand are typically manufactured by methods that are different fromconventional procedures for preparing somewhat larger, i.e., microscale,structures. Thus, although the exact boundaries of the class ofnanostructures are not defined by a particular numerical size limit, theterm has come to signify such a class that is readily recognized bythose skilled in the art. In many cases, an upper limit of the size ofthe dimensions that characterize nanostructures is about 500 nm.

Where the diameter of a nanoelement is below a certain amount, say 50nm, quantum confinement occurs where electrons can only move in thelength direction of the nanoelement; whereas for the diametral plane,the electrons occupy quantum mechanical eigenstates.

The electrical and optical properties of semiconductor nanowhiskers arefundamentally determined by their crystalline structure, shape, andsize. In particular, a small variation of the width of the whisker mayprovoke a considerable change in the separation of the energy states dueto the quantum confinement effect. Accordingly, it is of importance thatthe whisker width can be chosen freely, and, of equal importance, isthat the width can be kept constant for extended whisker lengths. This,together with the possibility of positioning whiskers at selectedpositions on a substrate, will be necessary if an integration of whiskertechnology with current semiconductor component technology is to bepossible. Several experimental studies on the growth of GaAs whiskershave been made, the most important reported by Hiruma et al. They grewIII-V nano-whiskers on III-V substrates in a metal organic chemicalvapor deposition—MOCVD-growth system—K. Hiruma, M. Yazawa, K. Haraguchi,K. Ogawa, T. Katsuyama, M. Koguchi, and H. Kakibayashi, J. Appl. Phys.74, 3162 1993; K. Hiruma, M. Yazawa, T. Katsuyama, K. Ogawa, K.Haraguchi, M. Koguchi, and H. Kakibayashi, J. Appl. Phys. 77, 4471995 E.I. Givargizov, J. Cryst. Growth 31, 20 1975; X. F. Duan, J. F. Wang, andC. M. Lieber, Appl. Phys. Lett. 76, 1116 2000; K. Hiruma, H. Murakoshi,M. Yazawa, K. Ogawa, S. Fukuhara, M. Shirai, and T. Katsuyama, IEICETrans. Electron. E77C, 1420 1994; K. Hiruma, et al, “Self-organisedgrowth on GaAs/InAs heterostructure nanocylinders by organometallicvapor phase epitaxy”, J. Crystal growth 163, (1996), 226-231. Theirapproach relied on annealing a thin Au film to form the seed particles.In this way, they achieved a homogeneous whisker width distribution, themean size of which could be controlled by the thickness of the Au layerand the way this layer transforms to nanoparticles. With this technique,it is difficult to control the size and surface coverage separately, andit is virtually impossible to achieve a low coverage. The correlationbetween film thickness and whisker thickness was not straightforward,since the whisker width also depended on growth temperature, and therewere even signs of a temperature-dependent equilibrium size of the Auparticles. The authors also noticed a strong correlation between thesize of the Au droplets de-posited from a scanning tunneling microscopetip and the resulting whisker width. For the free-flying Si whiskersgrown by Lieber et al.,—Y. Cui, L. J. Lauhon, M. S. Gudiksen, J. F.Wang, and C. M. Lieber, Appl. Phys. Lett. 78, 2214, 2001—a clearparticle-whisker size correlation has been shown.

It is necessary, if whiskers are to be used as electrical components,that there should be well-defined electrical junctions situated alongthe length of a whisker, and much work has been directed at achievingthis—see for example Hiruma et al, “Growth and Characterisation ofNanometer-Scale GaAs, AlGaAs and GaAs/InAs Wires” IEICE Trans.Electron., Vol. E77-C, No. 9 Sep. 1994, pp 1420-1424. However, muchimprovement is necessary.

Much work has also been carried out on carbon nanotubes (CNTs). Despiteprogress, research has been frustrated by a lack of control of theconductivity-type of CNTs and an inability to form 1D heterostructuresin a controlled manner. Randomly formed interfaces as kinks betweenmetallic and semiconducting parts of CNTs have been identified andstudied (Yao et al, Nature, 1999, 402, 273) as have doping (pn)junctions in semiconducting CNTs (Derycke et al, Nano Letters, 2001, 1,453) and transitions between CNTs and semiconductor (Si and SiC)nanowhiskers (Hu et al, Nature, 1999, 399, 48).

In a separate trend of development, attempts to fabricate 1D deviceshave been made since the late 1980s by top-down methods, as pioneered byRandall, Reed and co-workers at Texas Instruments—M. A. Reed et al.,Phys. Rev. Lett. 60, 535 (1988). Their top-down approach, which stillrepresents the state of the art for this family of quantum devices, isbased on epitaxial growth of multi-layers defining the two barriers andthe central quantum well. Electron-beam lithography is then used todefine the lateral confinement pattern, together with evaporation of themetallic layers to form the top contact. A lift-off process is then usedto remove the e-beam-sensitive resist from the surface, and reactive ionetching removes all the material surrounding the intended narrowcolumns. Finally, the devices are contacted via the substrate and fromthe top using a polyimide layer. In the studies of devices fabricated bythis bottom-up technique, 100-200 nm diameter columns have beenobserved, however, with rather disappointing electrical characteristicsand peak-to-valley currents at best around 1.1:1. An alternativeapproach to realizing low-dimensional resonant tunneling devices hasbeen reported more recently, employed strain-induced formation ofself-assembled quantum dots (I. E. Itskevich et al., Phys. Rev. B 54,16401 (1996); M. Narihiro, G. Yusa, Y. Nakamura, T. Noda, H. Sakaki,Appl. Phys. Lett. 70, 105 (1996); M. Borgstrom et al., Appl. Phys. Lett.78, 3232 (2001)).

SUMMARY OF THE INVENTION

The invention comprises a method of forming nanowhiskers,one-dimensional semiconductor nanocrystals, in which segments of thewhisker have different compositions, for example indium arsenidewhiskers containing segments of indium phosphide, wherein conditions forgrowth allow the formation of abrupt interfaces and heterostructurebarriers of thickness from a few monolayers to hundreds of nanometers,thus creating a one-dimensional landscape along which electrons canmove. In a preferred method of chemical beam epitaxy method (CBE), rapidalteration of the composition is controlled by the supply of precursoratoms into a eutectic melt of seed particle and substrate, supplied asmolecular beams into the ultra high vacuum chamber. The rapid switchingbetween different compositions is obtained via a sequence where growthis interrupted or at least reduced to an insignificant amount, andsupersaturation conditions for growth are reestablished; at least,change of composition and supersaturation is changed faster than anyappreciable growth. With abrupt changes in material of the whisker,stresses and strains arising from lattice mismatch are accommodated byradial outward bulging of the whisker, or at least by lateraldisplacement of the atoms in the lattice planes near the junction.

Further, the invention includes a technique for the synthesis ofsize-selected, epitaxial nano-whiskers, grown on a crystallinesubstrate. As catalysts, size-selected gold aerosol particles are used,which enables the surface coverage to be varied completely independentlyof the whisker diameter. The whiskers were rod shaped, with a uniformdiameter between 10 and 50 nm, correlated to the size of the catalyticseed. By the use of nano-manipulation of the aerosol particles,individual nano-whiskers can be nucleated in a controlled manner atspecific positions on a substrate with accuracy on the nm level. Themethod of the invention enhances width control of the whisker by virtueof choice of nanoparticle. The nanoparticle may be an aerosol or aliquid alloy on the substrate may be made by starting from goldrectangles formed on the substrate which when melted form accuratediameter balls. Other materials may be used instead of gold as the seedparticle, e.g. Gallium.

Whilst it is desirable in many applications to have nanowhiskers whichare essentially constant in diameter, the shape of the whisker, andother attributes, may be varied by selectively changing the diffusionconstant (diffusion coefficient) of the group III material, e.g. Ga,during whisker formation. This can be done by:

-   -   Lowering the temperature of the process—this produces whiskers        tapered towards their free ends;    -   Increasing the pressure of the group V material;    -   Increasing the pressure of both group V and group III materials.

More specifically, the invention provides a method of forming ananowhisker comprising:

depositing a seed particle on a substrate, and

exposing the seed particle to materials under controlled conditions oftemperature and pressure such as to form a melt with the seed particle,so that the seed particle melt rises on top of a column whereby to forma nanowhisker, the column of the nanowhisker having a diameter with ananometer dimension;

wherein during the growth of the column, selectively changing thecompositions of said materials whereby to abruptly change thecomposition of the material of the column at regions along its length,whilst retaining epitaxial growth, whereby to form a column having alongits length at least first and second semiconductor segment lengths, thefirst semiconductor segment being of a material having a different bandgap from that of the second semiconductor segment.

Functional 1D resonant tunneling diodes and other components andstructures have been obtained via bottom-up assembly of designedsegments of different semiconductor materials in III/V nanowhiskers.Electronic and photonics components comprising nanowhiskers have alsobeen formed as heterostructures, with a single crystal formation,wherein length segments of the nanowhisker are of different materials,so as to create well defined junctions in the whisker between differentband gap materials, whereby to create a component with a desiredfunction.

Thus, the invention provides in general terms a heterostructureelectronic or photonics component, comprising a nanowhisker having acolumn of a diameter with a nanometer dimension, the column havingdisposed along its length a plurality of length segments of differentmaterial composition with predetermined diametral boundaries betweenadjacent segments extending over a predetermined length of thenanowhisker column, such as to give desired band gap changes at theboundaries, in order to enable the component to carry out a desiredfunction.

In a general aspect, the invention provides an electronic or photoniccomponent, comprising a nanowhisker having a column with a diameter,which has a nanometer dimension,

the column comprising along its length at least first and second lengthsegments of different materials with an abrupt epitaxial compositionboundary disposed between the first and second segments, wherein latticemismatch at the boundary is accommodated by radial outward expansion ofthe nanowhisker at the boundary.

In another general aspect, the invention provides an electronic orphotonic component, comprising a nanowhisker having a column with adiameter, which has a nanometer dimension,

the column comprising along its length at least first and second lengthsegments of different materials with an abrupt epitaxial diametralmaterial boundary disposed between the first and second segments,wherein the transition between the composition of the differentmaterials of the first and second segments occurs over an axial distanceof not more than eight diametral lattice planes. Preferably, thetransition between the composition of the first and second segmentoccurs over an axial distance of not more than 6, lattice planes,preferably not more than 5 lattice planes, still more preferably notmore than 4 lattice planes, still more preferably not more than 3lattice planes, still more preferably not more than 2 lattice planes andmost preferably not more than one lattice plane.

In a further aspect, the invention provides an electronic or photoniccomponent, comprising a nanowhisker having a column with a diameterwhich has a nanometer dimension, the column comprising along its lengthat least first and second length segments of different materials, thefirst segment having a stoichiometric composition of the formA_(1-x)B_(x), and the second segment having a stoichiometric compositionof the form A_(1-y)B_(y), where A and B are selected substances, and xand y are variables, wherein an epitaxial composition boundary disposedbetween the first and second segments, comprises a predetermined gradualchange from the variable x to the variable y over a predetermined numberof diametral lattice planes. In a similar embodiment the compositions ofthe first and second segments of a nanowhisker of the invention can berepresented by the formulas and A_(1-x)B_(x)C, and A_(1-y)B_(y)C,respectively, wherein A and represent elements of one group, e.g., groupIII, of the periodic table, and C represents an element of anothergroup, e.g., group V, of the periodic table. The variables x and y mayassume a value between 0 and 1, and represent different numbers withinthat range. Thus, such a nanowhisker is formed of a compoundsemiconductor that may vary in composition along its length, therebyincorporating a heterojunction. An example of such a compoundsemiconductor is Al_(x)Ga_(1-x)As. A nanowhisker of the invention may beconstructed to have, e.g., two lengthwise segments, a first segmenthaving a composition Al_(1-x)Ga_(x)As, wherein the variable x has agiven value between 0 and 1 and a second segment having a compositionAl_(1-y)Ga_(y)As, wherein the variable y has a second value differentfrom the value of x. Between the two segments is an interface withinwhich the composition varies continuously from the composition of thefirst segment to that of the second segment, i.e., the value of thevariable x changes continuously, and usually monotonically, to the valueof the variable y. This interface thus constitutes a heterojunction. Thetransition may be made to occur over a predetermined number of diametrallattice planes by adjusting the conditions under which the whiskers aregrown, as will be explained in more detail below. Furthermore, thegrowth conditions can be periodically adjusted to produce a plurality ofsuch heterojunctions along the length of the nanowhisker.

The diameter of the nanowhisker is controlled by the invention to beessentially constant along the length of the nanowhisker, or having adefined variation, such as a controlled taper. This ensures preciseelectrical parameters for the nanowhisker, the controlled taper beingequivalent to producing a voltage gradient along the length of thenanowhisker. The diameter may be small enough such that the nanowhiskerexhibits quantum confinement effects. Although the diameter is preciselycontrolled, there will be small variations in the diameter arising fromthe processing method, in particular a radial outward bulging of thenanowhisker at a composition boundary in order to accommodate latticemismatch in the epitaxial structure. In addition the diameter of onesegment may be slightly different from that of another segment of adifferent material, because of the difference in lattice dimensions.

According to the invention the diameter of the nanowhiskers preferablywill not be greater than about 500 nm, preferably not greater than about100 nm, and more preferably not greater than about 50 nm. Furthermore,the diameter of the nanowhiskers of the invention may preferably be in arange of not greater than about 20 nm, or not great than about 10 nm, ornot greater than about 5 nm.

The precision of formation of the nanowhisker enables production ofdevices relying on quantum confinement effects, in particular a resonanttunneling diode. Thus, an RTD has been developed wherein the emitter,collector and the central quantum dot are made from InAs and the barriermaterial from InP. Ideal resonant tunneling behavior, withpeak-to-valley ratios of up to 50:1, was observed at low temperatures.

In a specific aspect, the invention provides a resonant tunneling diode,comprising a nanowhisker having a column of a diameter with a nanometerdimension, such as to exhibit quantum confinement effects,

the column comprising along its length first and second semiconductorlength segment forming respectively an emitter and a collector, and,disposed between the first and second semiconductor segments, third andfourth length segments of material having a different band gap from thatof the first and second semiconductor segments, and a fifth centrallength segment of a semiconductor material having a different band gapfrom that of the third and fourth segments, disposed between the thirdand fourth segments and forming a quantum well.

A problem which arises with an electrical or photonic component formedfrom a nanowhisker is that of making efficient electrical contacts tothe nanowhisker.

One method is to remove the nanowhisker from its substrate, by amechanical scraping process, and to deposit the nanowhiskers on afurther substrate, on their side lengthwise on the substrate. Metallisedbond pads may then be formed over the ends of the nanowhisker, oralternatively the nanowhisker can be manipulated to be positioned overpreformed contact pads.

Alternatively, in a method which may be better suited tomass-production, the nanowhiskers may be left on the substrate, withtheir base ends having been formed on an electrical contact. Onceformed, the nanowhiskers may be encapsulated in a resin or glassysubstance, and then contact pads may be formed over the surface of theencapsulation in contact with the free ends of the nanowhiskers. Toassist in this, the catalytic particle melt, towards the end of theformation of the nanowhisker, may have extra conductive substancesinjected into it, so as to improve the electrical contact with the bondpads.

Further specific components are set forth in the appended claims, anddescribed below. In particular, these include a heterobipolartransistor, and light emitting diodes and photodetectors.

Light emitting diodes are well suited to the present invention, since itis possible to construct them with an emission wavelength which can beselected at will from a continuous range of wavelengths over the UV,visible, and infrared regions.

The present invention provides a light emitting diode, comprising ananowhisker having a column of a diameter with a nanometer dimension,such as to exhibit quantum confinement effects,

the column comprising along its length in sequence first, second andthird semiconductor length segments comprising respectively an emitter,quantum well active segment and collector, said second segment having adifferent band gap from that of the first and second segments, andforming an active area of the light emitting diode.

One particular application of a light emitting diode is for emission ofsingle photons. This is of use in various applications, but inparticular in quantum cryptography, where unauthorised interception of aphoton stream will inevitably cause destruction or modification of thephoton, in accordance with quantum theory, and thus corruption of thetransmitted signal—see P. Michler, A. Imamoglu, M. D. Mason, P. J.Carson, G. F. Strouse, S. K. Buratto, Nature 406, 968 (2000); C.Santori, M. Pelton, G. Solomon, Y. Dale, Y. Yamamoto, Phys. Rev. Lett.86, 1502 (2001).

The invention provides a single photon light source, comprising a onedimensional nanoelement, having disposed along its length a volume ofoptically active material sufficiently small to form a quantum well,with tunneling barriers formed on either side of the quantum well, suchthat in use the quantum well is capable of emitting a single photon at atime.

Another form of light source in accordance with the invention isdesigned for terahertz radiation, beyond the far infrared. Much work hasbeen done on superlattices, pioneered by Capasso and co-workers atLucent Technologies. Their ‘quantum cascade’ lasers utilise intersubbandphoton emission in InGaAs/InAlAs/InP heterostructures, and have achievedroom temperature (pulsed mode) operation at wavelengths up to 17microns. See for example IEEE Spectrum July 2002, pages 23, 24, “UsingUnusable Frequencies” and F. Capasso, C. Gmachl, D. L. Sivco, and a. Y.Cho, “Quantum cascade lasers” Physics Today, May 2000, pp. 34-39.

The invention provides a source of terahertz radiation, comprising ananowhisker having a column of a diameter with a nanometer dimension,the column including a multiplicity of layers of a first band gapsemiconductor interleaved with a multiplicity of layers of a second bandgap material, whereby to form a superlattice, the dimensions being suchthat electrons can move with a wave vector such as to radiate terahertzradiation.

In components, structures and processes according to the invention, anarray of a large number of nanowhiskers may be formed extending from asubstrate, essentially parallel to one another. There are variousmethods of forming such arrays, for example positioning an array ofaerosol particles on the substrate to provide catalytic seed particles,depositing particles on the substrate from a colloidal solution, orforming on the substrate by a nanoimprint lithography (NIL) process (orby any other lithography process, e.g. e beam, UV, or X-ray), an arrayof areas of predetermined shape (rectangular or other shape) andthickness, which when heated, form balls of a desired volume to permitthe nanowhisker growth process to proceed.

Such arrays may be employed as photonic crystals, solar cells comprisedof a large number of photodetectors, field emission displays (FED),converters to convert an infrared image to a visible light image, all asdescribed herein below. A further application is that of a polarisationfilter.

In processes of the invention, an array of a large number ofnanowhiskers may be employed to create a layer of an epitaxial materialon a wafer substrate of a cheaper substance, for example silicon. Along-standing problem in the art is the formation of single crystalwafers of expensive III-V materials, from which chips can be formed.Much research has been made into forming single crystal layers onsilicon wafer substrates—see for example WO 02/01648. However furtherimprovements are desirable.

In accordance with the invention, a substrate of silicon or othersubstance is provided on which is grown a mask material, resistant toepitaxial growth, for example a dielectric material such as SiO₂, orSi₃N₄. An array of nanometer-dimensioned apertures is formed in the maskmaterial, such as by a NIL process, and catalytic seed-forming materialis deposited in the apertures. Alternatively an array of seed formingmaterial areas is deposited on the substrate, and a layer of maskmaterial is then deposited over the substrate and the seed particleareas. Application of heat causes melting of the seed particle areas tocreate the seed particles, and then growth of the nanowhiskers of thedesired III-V or other material is initiated. After growth of thenanowhiskers, growth of the desired material continues, using thewhiskers as nucleation centres, until a single continuous layer of thematerial is formed. The material is single crystal epitaxial. Aspreferred, the seed particle melt at the end of the nanowhiskers isremoved at a convenient opportunity to avoid contamination of theepitaxial layer.

In a modification, mass growth of the epitaxial layer is initiated,using the seed particle melts as nucleation points, prior to formationof the nanowhiskers, and while the growth underneath the seed particlesis still in the liquid phase.

In a further modification, microscopic V-grooves are formed in the uppersurface of the silicon surface, for example <111> etchings in a <100>substrate. The seed particle forming areas are formed on the surfaces ofthe V-grooves, whereby the nanowhiskers grow at an angle to thesubstrate, and cross one another at the grooves. This makes for a moreefficient growth of the epitaxial layer from the nanowhisker nucleationcentres. Further, grain boundaries between domain areas with differentgrowth phases are avoided; which has been a problem with priorprocesses.

The present invention thus provides in a further aspect a method forforming an epitaxial layer of a desired material on a substrate of adifferent material, the method comprising forming on a substrate aconfiguration of seed particle material areas, forming a layer of maskmaterial around the seed particle areas, growing nanowhiskers from theseed particles areas of said desired material, and continuing to growsaid desired material, using the nanowhiskers as growth sites, wherebyto create an epitaxial layer of said desired material extending oversaid substrate.

In a further aspect of the invention, processes have been developed forforming nanowhiskers of III-V material extending in the <100> direction,as opposed to the usual <111> direction for nanowhiskers. This hasimportant applications, particularly for nitride materials which tend togrow in the <111> direction, but with many stacking faults, as thematerial alternates between a zinc blende and wurtzite structure.

The invention provides a method of forming nanowhiskers comprisingproviding a substrate, forming a configuration of seed particles on theupper surface, growing nanowhiskers from said seed particles whichextend from the substrate initially in a <111> direction, and forming ashort segment of a barrier material in said nanowhiskers such as tochange their direction of growth to a <100> direction.

In a further aspect, the invention provides method of formingnanowhiskers, a method of forming nanowhiskers, comprising providing asubstrate, forming a configuration of seed particles on the uppersurface, growing nanowhiskers from said seed particles which extend fromthe substrate initially in a <111> direction, and changing the growthconditions of said nanowhiskers such as to change their direction ofgrowth to a <100> direction.The present invention also relates to one-dimensional nanoelementsincorporated in MEMS devices—micromechanical devices.

In one aspect a substrate, for example of silicon, has a matrix ofelectrical contact areas formed on one surface. On each contact area,one, or a number, of nanowhiskers are formed from, for example, goldcatalyst particles so as to be upstanding from the substrate's surface.Each nanowhisker, or group of nanowhiskers may therefore be individuallyaddressable by electrical signals. Such a structure may make contactwith the end of a nerve or perhaps the nerves in the retina of an eye,and the electrodes may be activated so as to provide a repairing orartificial function for enabling the nerve. Thus for example, whenapplied in the retina of an eye, the structure may overcome certainblindness problems.

In another aspect a nanowhisker is provided, which may function as anerve electrode or in other applications, wherein the whisker is formedof silicon or of a metal which may be oxidised, and the whisker isoxidised to form a layer of oxide along its length. The particle melt atthe end of the whisker however including gold or other non-oxidisablematerial remains free of oxide and may therefore be used to form anelectrical contact. This arrangement provides more precise electricalcharacteristics than nanowhiskers with exposed conductive material alongtheir lengths and such nanowhiskers may be used as nerve electrodes oras devices where the capacitance of the nanowhisker is of importance. Asan alternative, other materials may be used as the outer layer forexample higher bandgap shells, for example where the whisker is formedof gallium arsenide, the outer layer may be gallium phosphide.

An important application of nanostructures is in micromechanicalcantilever beams where a beam fixed at one end projects into space andmay be subject to an external force, for example, electrical or weightor an external object or a chemical force, to give a bending of thecantilever. This bending may be detected for example by a change inelectrical capacitance of the structure.

In a further aspect the present invention provides one or morenanowhiskers, which may or may not be oxidised in accordance with theabove-mentioned aspect of the invention along their length to provide acantilever or an array of cantilevers formed as a row or parallel beams.Such an arrangement may provide an order of magnitude or moresensitivity than a previous arrangement where an etching process hasbeen used to produce the beams.

One application for such cantilevers is where the whiskers are formedwith a material with a coating which is sensitive to certain organicmolecules or biological molecules, such that a molecule, when makingcontact with a cantilever beam undergoes a certain chemical reaction.This produces certain stresses on the cantilever beam and causes bendingof the beam, which may be detected by optical or electrical monitoring.

In a further specific aspect, a nanowhisker is formed on a substrateprojecting up into an aperture of a layer of material, which isessentially insulative. The upper surface of the insulative layer has anelectrically conductive material formed thereon. This electricallyconductive material is roughly the same height from the substrate as thetip of the nanowhisker, which has a conductive seed particle meltthereon. By appropriate activation of the conductive material, thewhisker may be made to mechanically vibrate within the aperture at acertain eigen frequency, for example, in the gigahertz range. During theperiod of a single vibration, a single electron is transferred from oneside of the conductive material to the other via the seed particle melt.This creates a current standard generator, where the current I throughthe conductive material is equal to product of the frequency ofvibration and the charge e of an electron: I=f·e.

If the whisker is sensitised to attract molecules of a certain type,then the deposition of a molecule onto the whisker will change theinertial characteristics of the whisker and therefore its naturalfrequency of vibration. This may therefore be detected by electricalactivation of the conductive material. This technique may be used tocalculate the weight of a molecule to a very accurate degree.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be now be described merelyby way of example with reference to the accompanying drawings, in which:

FIG. 1 is a schematic depiction of a fabrication technique according tothe invention, for forming nanowhiskers: (a) deposition of size-selectedAu aerosol particles onto a GaAs substrate; (b) AFM manipulation of theparticles for the positioning of the whiskers; (c) alloying to make aeutectic melt between Au and Ga from the surface; (d) GaAs whiskergrowth.

FIG. 2. (a) TEM micrograph of GaAs nano-whiskers grown from 10 nm Auaerosol particles. (b) SEM micrograph of a GaAs<111>B substrate withGaAs whiskers grown from 40 nm Au aerosol particles. (c) High-resolutionelectron microscope image 400 kV of GaAs whisker grown from an Aucluster. The inset shows a magnified part of the whisker.

FIG. 3 is a schematic diagram of apparatus for carrying out the methodsof this invention.

FIG. 4. Composition profile of an InAs nanowhisker, according to anembodiment of the invention, containing several InP heterostructures,using reciprocal space analysis of lattice spacing. (a) High-resolutionTEM image of a whisker with a diameter of 40 nm. (b) Power spectrum ofthe image in (a). (c) An inverse Fourier transform using the informationclosest to the InP part of the split 200 reflection. InP (bright) islocated in three bands with approximately 25, 8 and 1.5 nm width,respectively. (d) Superimposed images, using an identical mask over theInP and InAs parts of the 200 reflection, respectively.

FIG. 5. Analysis of InP heterostructures inside InAs nanowhiskers. (a)TEM image of InP barriers (100, 25, 8, and 1.5 nm) inside a 40 nmdiameter InAs nanowhisker. (b) Magnification of the 8 nm barrier region,showing crystalline perfection and the interface abruptness on the levelof monolayers. (c) Simulated band-structure diagram of the InAs/InPheterostructures, including (left edge) ideal formation of ohmiccontacts to InAs. (d) Ohmic I-V dependence for a homogeneous InAswhisker, contrasted by the strongly nonlinear I-V behavior seen for anInAs whisker containing an 80 nm InP barrier. (e) Arrhenius plot showingmeasurements of thermionic excitation of electrons across the InPbarrier (at a bias of 10 mV), yielding a barrier height of 0.57 eV.

FIG. 6. Evaluation of transport mechanisms for single barriers ofvarious thick messes, for use in resonant tunneling diodes of theinvention. (A) A SEM image of a whisker on the growth substrate. (Scalebar depicts 1 μm.) (B) An InAs/InP nanowhisker contacted by two alloyedohmic contacts. (Scale bar depicts 2 μm.) (C) TEM image of an InAswhisker with an 8 nm InP segment perpendicular to the long axis of thewhisker. (D) The current-voltage characteristics for three differentbarrier situations;

FIG. 7. High-resolution TEM imaging. (A) A TEM image of an InAs whiskergrown in the <111> direction with two InP barriers for forming a firstembodiment of the invention. (Scale bar depicts 8 nm.) (B) Aone-dimensionally integrated profile of the boxed area in A. The widthof the barrier is about 5.5 nm (16 lattice spacing), and the interfacesharpness is of the order of 1-3 lattice spacings, judged by the jump inimage contrast.

FIG. 8. A resonant tunneling diode (RTD) forming an embodiment of theinvention.

(A) TEM image of the top end of a whisker with the double barrierclearly visible, in this case with a barrier thickness of about 5 nm(scale bar depicts 30 nm).

(B) The principle of the energy band diagram for the device investigatedwith the characteristic electronic states in the emitter regionindicated (left).

(C) Current-voltage data for the same device as shown in A and Brevealing a sharp peak in the characteristics, reflecting resonanttunneling into the ground state, E1z, with a voltage width of about 5mV. This width can be translated into an energy width of the transitionof about 2 meV, corresponding to the width of the shaded energy band inthe emitter from which electrons tunnel. The device characteristics areshown in the inset, which provides a magnified view of the resonancepeak for increasing voltage and for decreasing voltage.

FIG. 9 is a schematic representation of the preferred embodiment of theresonant tunneling diode according to the invention;

FIG. 10 is a schematic representation of a further embodiment of theinvention including a wide band gap insulating segment;

FIG. 11 is a schematic representation of a further embodiment of theinvention comprising a hetero bipolar transistor (HBT);

FIG. 12 is a band gap diagram of the HBT correlated with the HBTstructure;

FIG. 13 is a diagram showing band gap variation with compositionalchange of a ternary compound;

FIGS. 14A and 14B are diagrams showing band gap versus latticedimensions for a variety of semiconductor compounds;

FIG. 15 is a schematic representation of an embodiment of the inventioncomprising a light emitting diode and laser;

FIG. 16 is a schematic representation of a further embodiment of theinvention comprising the application of a laser to detection ofindividual molecules of desired species;

FIG. 17 is a schematic representation of a further embodiment of theinvention comprising the application of an array of lasers to patterningphotoresists in a NIL process;

FIG. 18A is a schematic representation of a further embodiment of theinvention comprising a photodetector, and FIGS. 18B and 18C are variantsthereof;

FIG. 19A is a schematic representation of a further embodiment of theinvention comprising a solar cell, and FIG. 19B is a variant thereof;

FIG. 20 is a schematic representation of a further embodiment of theinvention comprising a radiation source of terahertz radiation;

FIGS. 21A-C are schematic representations for explaining an embodimentof the invention comprising a photonic crystal, and FIG. 21D is avariant thereof for forming a 3-D photonics crystal;

FIGS. 22A-G are schematic representations of a further embodiment of theinvention for forming a layer of material epitaxial with a substrate,wherein the lattices are not matched to one another;

FIGS. 23A-C are schematic representations for explaining a furtherembodiment of the invention for forming a layer of material epitaxialwith a substrate, wherein the lattices are not matched to one another;

FIGS. 24A-B are schematic representations for explaining a furtherembodiment of the invention, for forming whiskers, which extend ina<100> direction, as opposed to the usual <111> direction;

FIGS. 25A-B are schematic representations of a further embodiment of theinvention comprising a field emission display (fed), wherein theindividual elements of the display are nanowhiskers and are individuallyaddressable;

FIG. 26 is a schematic representation of a further embodiment of theinvention comprising an arrangement for upconverting an image in theinfrared region to a visible light region;

FIG. 27 is a schematic representation of a further embodiment of theinvention comprising an antenna for infrared radiation;

FIG. 28 is a schematic representation of a further arrangementcomprising a ferromagnetic whisker for spintronics applications;

FIG. 29 is a schematic view of a further embodiment of the inventioncomprising an array of selectively addressable electrodes forimplantation into a nerve;

FIG. 30 is a schematic view of a further embodiment of the inventioncomprising a nanowhisker with an oxidised outer surface along itslength;

FIG. 31 is a schematic view of a further embodiment comprising a row ofnanowhiskers upstanding from a substrate and forming a cantileverarrangement;

FIG. 32 is a schematic view of a further embodiment of the inventioncomprising a nanowhisker arranged for oscillation and providing precisemeasurements of weight and frequency; and

FIG. 33 is a schematic view of a further embodiment of the invention,comprising the tip of a Scanning Tunneling Microscope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Methods of producing nanowhiskers, in accordance with the invention willnow be described. Such methods may be suitable for production ofresonant tunneling diodes described below, and other electronic and/orphotonic components, as will become apparent.

Whiskers are highly anisotropic structures, which are spatiallycatalyzed by molten metallic droplets, often unintentionally introducedas contaminants, on a crystal surface. Gold is commonly chosen ascatalyst, or seed particle, since it forms eutectic alloys withsemiconductor materials or constituents such as Si, Ga, and In. Themelting points of these eutectic alloys are lower than the usual growthtemperatures for Si and group III-V materials. The molten metal dropletserves as a miniature, liquid phase, epitaxy system, where precursorsare fed into it in the form of a vapour or, in this case, by a molecularbeam in vacuum. The growth is usually referred to as vapour-liquid-solidgrowth. The electrical and optical properties of semiconductornanowhiskers are fundamentally determined by their crystallinestructure, shape, and size. In particular, a small variation of thewidth of the whisker provokes a considerable change in the separation ofthe energy states due to the quantum confinement effect. Accordingly, itis of importance that the whisker width can be chosen freely, and, ofequal importance, is that the width can be kept constant for extendedwhisker lengths. This, together with the possibility of positioningwhiskers at selected positions on a substrate, is necessary for anintegration of whisker technology with current semiconductor componenttechnology.

According to the invention, techniques have been developed for thesynthesis of size-selected, epitaxial nanowhiskers, grown on acrystalline substrate. The Chemical Beam Epitaxy apparatus employed inthe techniques described below is shown schematically in FIG. 3.

Chemical Beam Epitaxy (CBE) combines a beam epitaxial technique likeMolecular Beam Epitaxy (MBE) and the use of chemical sources similar toMetal Organic Chemical Vapor Deposition (MOCVD). In MOCVD or relatedlaser ablation techniques, the pressure inside the reactor is usuallygreater than 10 mbar and the gaseous reactants are viscous, which meansthat they have a relatively high resistance to flow. The chemicals reachthe substrate surface by diffusion. CBE reduces the pressure to lessthan 10⁻⁴ mbar and the mean free path of the diffusants then becomeslonger than the distance between the source inlet and the substrate. Thetransport becomes collision free and occurs in the form of a molecularbeam. The exclusion of the gas diffusion in the CBE system means a fastresponse in the flow at the substrate surface and this makes it possibleto grow atomically abrupt interfaces.

The CBE apparatus shown in FIG. 3 consists of a UHV (ultra-high vacuum)growth chamber 1001 where the sample 1021 is mounted on a metal sampleholder 1041 connected to a heater 1061. Around the chamber there is aring 1081 filled with liquid nitrogen that is called the cryoshroud. Thecryoshroud pumps away species that don't impinge or that desorb from thesubstrate surface. It prevents contamination of the growing surfacelayer and reduces the memory effect. Vacuum pumps 1101 are provided.

The sources 1121 for CBE are in liquid phase and they are contained inbottles which have an overpressure compared to the chamber. The sourcesare usually as follows: TMGa, TEGa, TMIn, TBAs, TBP. The bottles arestored in constant-temperature baths and by controlling the temperatureof the liquid source, the partial pressure of the vapor above the liquidis regulated. The vapor is then fed into the chamber through a pipecomplex 1141 to, in the end of the pipe just before the growth chamber,a source injector 1161. The source injector is responsible for injectionof the gas sources into the growth chamber 1001, and for generation of amolecular beam with stable and uniform intensity. The III-material, fromthe metal organic compounds TMIn (trimethylindium), TMGa(trimethylgallium) or TEGa (triethylgallium), will be injected by lowtemperature injectors to avoid condensation of the growth species. Theywill decompose at the substrate surface. The V-material is provided bythe metal-organic compounds, TBAs (tertiarybutylarsine) or TBP(tertiarybutylphosphine). As opposed to the decomposition of theIII-material, the V-material will be decomposed before injection intothe growth chamber 1001, at high temperatures, in the injectors 1161.Those injectors 1161 are called cracking cells and the temperatures arekept around 900° C. The source beam impinges directly on the heatedsubstrate surface. Either the molecule gets enough thermal energy fromthe surface substrate to dissociate in all its three alkyl radicals,leaving the elemental group III atom on the surface, or the molecule getdesorbed in an undissociated or partially dissociated shape. Which ofthese processes dominates depends on the temperature of the substrateand the arrival rate of the molecules to the surface. At highertemperatures, the growth rate will be limited by the supply and at lowertemperatures it will be limited by the alkyl desorption that will blocksites.

This Chemical Beam Epitaxy method permits formation of heterojunctionswithin a nanowhisker, which are abrupt, in the sense there is a rapidtransition from one material to another over a few atomic layers.

For the purposes of this specification, “atomically abruptheterojunction”, is intended to mean a transition from one material toanother material over two or less atomic monolayers, wherein the onematerial is at least 90% pure on one side of the two monolayers, and theother material is at least 90% pure on the other side of the twomonolayers. Such “atomically abrupt heterojunctions” are sufficientlyabrupt to permit fabrication of heterojunctions defining quantum wellsin an electrical component having a series of heterojunctions andassociated quantum wells.

For the purposes of this specification, “sharp heterojunction”, isintended to mean a transition from one material to another material overfive or less atomic monolayers, wherein the one material is at least 90%pure on one side of the five monolayers, and the other material is atleast 90% pure on the other side of the five monolayers. Such “sharpheterojunctions” are sufficiently sharp to permit fabrication ofelectrical components having one, or a series of, heterojunctions withina nanoelement, where the heterojunctions have to be accurately defined.Such “sharp heterojunctions” are also sufficiently sharp for manycomponents relying on quantum effects.

As an illustration, in a compound AB used in the nanowhiskers of theinvention, where A represents one or more selected elements of a firstgroup and B represents one or more selected elements of a second group,the total proportions of the selected element(s) in the first group andthe selected element(s) in the second group are predetermined toconstitute a semiconductor compound designed to provide desiredproperties. The compound AB is considered 90% pure when the totalproportion of the selected element(s) within each group is at least 90%of its predetermined proportion.

Example 1

FIGS. 1 and 3 show whiskers of predetermined sizes grown from severalIII-V materials, in particular, GaAs whiskers with widths between 10 and50 nm. These whiskers can be grown rod shaped with a uniform diameter,in contrast to earlier reports on epitaxially grown nano-whiskers, whichtended to be tapered, narrowing from the base towards the top. Ascatalysts, size-selected gold aerosol particles were used, whereby thesurface coverage can be varied independently of the whisker diameter.

The whisker width in general is slightly larger than the seed particlediameter. This is mainly due to two factors: First, the gold particleincorporates Ga and possibly As from the substrate, which makes theparticle grow. Second, when the particle melts, the base diameter of theliquid cap will be determined by the wetting angle between the alloy andthe substrate surface. Simple assumptions give a widening of up to 50%,depending on temperature and particle diameter, and introduce areproducible correlation between the particle diameter and the width ofthe whisker.

A GaAs<111>B substrate 10 was used, etched in HCL:H₂O, 1:10 to removeany native oxide and surface contaminants before aerosol deposition. Thesize-selected Au particles 12 were made in a locally constructed aerosolfacility situated in a glove box 14 with ultra pure N₂ atmosphere. Theparticles are created in a tube furnace 16 by theevaporation/condensation method, at a temperature of about 1750° C., andare electrically charged by UV light at 18. The particles are sizeselected by means of a differential mobility analyzer DMA 20. The DMAclassifies the sizes of charged aerosol particles by balancing their airresistance against their mobility in an electric field. After sizeclassification, the particles were heated to 600° C., in order to makethem compact and spherical. The setup gives an aerosol flow with anarrow size distribution, the standard deviation being <5% of the meanparticle diameter. Still charged, the particles were deposited on thesubstrate 10 by means of an electric field E. Size-selected aerosolparticles in the range between 10 and 50 nm were used to grow whiskers.

After aerosol deposition, some samples were transferred to an AFMTopometrix Explorer 24, also situated inside the glove box, andconnected to the aerosol fabrication equipment. Thus, these samples wereexposed only to sub-ppm levels of H₂O and O₂ during the deposition andmanipulation phases. With the AFM tip, specific particles 12 wereselected and placed in a predetermined configuration, giving completecontrol of the positioning of individual seed particles.

The GaAs substrate 10 with Au aerosol particles 12, either arranged oras deposited, was then transferred into a chemical beam epitaxy CBEchamber. In a CBE configuration, GaAs growth occurs undervacuum/molecular beam conditions and with metal organic sources, in thiscase, triethylgallium TEG and tertiarybutylarsine TBA. The TBA isthermally pre-cracked to predominantly As₂ molecules, while the TEGusually cracks after impinging on the surface of the substrate. Thegrowth is typically performed with a slight As₂ over-pressure, whichmeans that the Ga flow determines the growth rate. Just before growth,the substrate was heated by a heater to 600° C. for 5 min, while exposedto an As₂ beam. In this step, the Au droplet can form an alloy with theGaAs constituents, whereby the Au particle absorbs some of the Ga fromthe substrate. The Au/Ga alloy forms at 339° C. However, this step alsoworks as a deoxidizing step, taking away any new native oxide layer,originating from the transport to and from the glove box system. Theoxide is expected to evaporate at 590° C., although this is not alwaysthe case. The volatility of the oxide can be followed with reflectivehigh-energy electron diffraction RHEED. With a successful transfer, astreaky diffraction pattern, indicating a crystalline, reconstructedsurface, can be seen already at temperatures lower than 500° C. Often,however, the oxide stays stable up to 590° C., sometimes as high as 630°C. The whisker growth was performed at substrate temperatures between500 and 560° C., with a TEG pressure of 0.5 mbar and a TBA pressure of2.0 mbar. After growth, the samples were studied by scanning andtransmission electron microscopy SEM and TEM.

The resulting whiskers were rod shaped and fairly homogeneous in size,although their lengths varied slightly. The size homogeneity was clearlydependent on the volatility of the surface oxide. For samples with ahard oxide, as seen with RHEED, the size homogeneity was decreased. Anoxygen-free environment is therefore to be preferred for reproducibleresults. At the growth temperatures described, no tapering of thewhiskers was observed, irrespective of particle size. For whiskers grownbelow 500° C., however, there were clear signs of tapering. The growthof either rod-shaped or tapered whiskers, depending on temperature, isexplained by the absence or presence of uncatalyzed growth on thesurfaces parallel to the long axis of the whisker. The simplest surfacesof this orientation are <110> facets. Under ordinary CBE growthconditions, close to the ones used in these experiments, <110> facetsare migration surfaces. However, at lower temperature, the Ga diffusionconstant decreases, which initiates growth on the <110> facets. In MOCVDgrowth the Ga migration length is even smaller, which explains thetypically tapered whiskers of prior workers.

In FIG. 2 a, a TEM image of a truss of 10±2-nm-wide whiskers grown from10 nm particles is shown. The relatively low density of whiskers isilluminated by the SEM image in FIG. 2 b, which is of a GaAs<111>Bsubstrate with GaAs whiskers grown from 40 nm Au aerosol particles. InFIG. 2 c, a single 40-nm-wide whisker is shown in a high-resolution TEMmicrograph. The growth direction is perpendicular to the close-packedplanes, i.e., 111 in the cubic sphalerite structure, as found by othergroups. Twinning defects and stacking faults can also be observed, wherethe whisker alternates between cubic and hexagonal structure. Most ofthe whisker has the anomalous wurzite structure W, except for the partclosest to the Au catalyst, which always is zinc blende Z. SF=stackingfault, T=twin plane. The change in image contrast at the core is due tothe hexagonal cross-section.

Such a growth method is used in the method described below withreference to FIGS. 4 to 6 for forming whiskers with segments of thewhisker with different compositions. The method is illustrated by Inkswhiskers containing segments of InP.

Example 2

Conditions for growth of nanowhiskers allow the formation of abruptinterfaces and heterostructure barriers of thickness from a fewmonolayers to 100 s of nanometers, thus creating a one-dimensionallandscape along which the electrons move. The crystalline perfection,the quality of the interfaces, and the variation in the lattice constantare demonstrated by high-resolution transmission electron microscopy,and the conduction band off-set of 0.6 eV is deduced from the currentdue to thermal excitation of electrons over an InP barrier.

In this method, the III-V whiskers are grown by the vapor-liquid-solidgrowth mode, with a gold nanoparticle catalytically inducing growth, inthe manner described above. Growth occurs in an ultrahigh vacuum chamber100, FIG. 3, designed for chemical beam epitaxy (CBE). The rapidalteration of the composition is controlled by the supply of precursoratoms into the eutectic melt, supplied as molecular beams into theultrahigh vacuum chamber. The rapid switching between differentcompositions (e.g., between InAs and InP) is obtained via a sequencewhere growth is interrupted as the indium source (TMIn) is switched off,followed by a change of the group III sources. Finally, thesupersaturation conditions, as a prerequisite for reinitiation ofgrowth, are reestablished as the indium source is again injected intothe growth chamber.

For the abruptness of the interfaces, FIG. 4 shows TEM analysis of anInAs whisker containing several InP heterostructure barriers. In FIG. 4a, a high-resolution image of the three topmost barriers is shown,recorded with a 400 kV HRTEM (point resolution 0.16 nm). FIG. 4 b showsa nonquadratic power spectrum of the HREM image, showing that the growthdirection is along [001] of the cubic lattice. The reflections show aslight splitting due to the difference in lattice constants between InAsand InP. FIG. 4 c shows an inverse Fourier transform, using a soft-edgemask over the part of the 200 reflection arising from the InP lattice. Acorresponding mask was put over the InAs part of the reflection. The twoimages were superimposed as in FIG. 4 d.

FIG. 5 a shows a TEM image of an InAs/InP whisker. The magnification ofthe 5 nm barrier in FIG. 5 b shows the atomic perfection and abruptnessof the heterostructure interface. Aligned with the 100 nm thick InPbarrier, the result of a 1D Poisson simulation (neglecting lateralquantization, the contribution of which is only about 10 meV) of theheterostructure 1D energy landscape expected to be experienced byelectrons moving along the whisker is drawn (FIG. 5 c). This gives anexpected band offset (q¼B) in the conduction band (where the electronsmove in n-type material) of 0.6 eV. This steeplechase-like potentialstructure is very different from the situation encountered for electronsin a homogeneous InAs whisker, for which ohmic behavior (i.e., a lineardependence of the current (I) on voltage (V)) is expected and indeedobserved (indicated curve in FIG. 5 d). This linear behavior isdramatically contrasted by the indicated I-V curve measured for an InAswhisker containing an 80 nm thick InP barrier. Strongly nonlinearbehavior is observed, with a voltage bias of more than 1V required toinduce current through the whisker. This field-induced tunnel currentincreases steeply with increasing bias voltage, as the effective barrierthrough which the electrons must tunnel narrows. To test whether theideal heterostructure band diagram within the 1D whisker is valid, thetemperature dependence of the current of electrons overcoming the InPbarrier via thermionic excitation was measured. The result is shown inFIG. 5 e, where the logarithm of the current (divided by T²) is plottedas a function of the inverse of the temperature in an Arrhenius fashion,measured at a small bias voltage (V) 10 mV) to minimize band-bendingeffects and the tunneling processes described above. From the slope ofthe line fitted to the experimental data points an effective barrierheight, q¼B, of 0.57 ev may, be deduced, in good agreement with thesimulation.

An added benefit of this approach to realizing heterostructures within1D whiskers is the advantageous condition for combining highlymismatched materials, provided by the efficient strain relaxation by theproximity to the open side surface in the whisker geometry. Incomparison, only a few atomic layers may be epitaxially grown intransitions between materials like InAs and InP with different latticeconstants before either islanding or misfit dislocations occur, therebypreventing formation of ideal heterointerfaces.

Resonant Tunneling Diodes and Heterobipolar Transistors

The present invention also comprises, at least in preferred embodiments,functional 1D (one-dimensional) resonant tunneling diodes (RTDs)obtained via bottom-up assembly of designed segments of differentsemiconductor materials in III/V nanowires. Such RTDs comprise, inorder, an emitter segment, a first barrier segment, quantum wellsegment, a second barrier segment, and a collector segment. As is knownto those skilled in the art, the barrier segments in RTDs are made thinenough that significant quantum tunneling of charge carriers is possibleunder conditions that favor such tunneling. In RTDs according to theinvention, fabricated in nanowitres, the nanowhiskers may be made thinenough so that the central quantum well is effectively a quantum dot. Ina concrete example, the emitter, collector and the central quantum dotmay be made from InAs and the barrier material from InP. In an example,excellent resonant tunneling behavior, with peak-to-valley ratios of upto 50:1, was observed.

According to the invention 1D heterostructure devices were fabricatedutilizing semiconductor nanowhiskers. The whiskers were grown by avapor-liquid-solid growth mode, size controlled by, and seeded from, Auaerosol particles, as more fully described above in Examples 1 and 2.Growth takes place in a chemical beam epitaxy chamber underultra-high-vacuum conditions where the supersaturation of the eutecticmelt between the Au particles and the reactants acts as the drivingforce for whisker growth.

The incorporation of heterostructure segments into the whiskers isachieved via the following switching sequence (more fully describedabove); the group III-source beam is switched off to stop growth, andshortly thereafter the group V-source is changed. Once the groupIII-source is reintroduced into the chamber, the supersaturation isre-established and growth continues. In examples described below thematerial system used was InAs for the emitter, collector and dot, andInP as the barrier material. The aerosol particles were chosen so thatthe final whisker diameter was 40-50 nm. In order to prepare contactedelectronic devices with single nanowhiskers as the active elements, thewhiskers were transferred from the growth substrate to a SiO₂-cappedsilicon wafer, on top of which large bond pads were predefined by Aumetal evaporation through a transmission electron microscope (TEM) gridmask. In FIG. 6B a scanning electron microscope (SEM) image of ananowire device is shown, displaying the alignment capability in thee-beam lithography system, allowing positioning of metallic electrodeson the nanowires with an accuracy that is better than 100 nm. FIG. 6Dshows the current-voltage (I-V) characteristics of a set ofsingle-barrier devices, as the thickness of the InP barrier was variedfrom 80 nm down to zero. The thicker InP segments act as ideal tunnelingbarriers for electron transport, allowing only thermal excitation overthis barrier (measured to be about 0.6 eV (23)) or tunneling madepossible by the effective thinning of the barrier when a large bias isapplied to the sample. In FIG. 6D it can be seen that almost no currentflows through the thick InP barrier. In samples containing thinnersingle barriers (FIG. 2C), quantum tunneling is possible and electronscan penetrate barriers thinner than about 10 nm in thickness. In theextreme case with zero barrier thickness, the I-V characteristics areperfectly linear down to at least 4.2 K. In order to verify thecrystalline quality and to evaluate the abruptness of theheterointerfaces high-resolution TEM investigations were performed. InFIG. 7A a magnification of a 5.5 nm thick InP barrier in a <111>-InAsnanowhisker is shown, where the (111) lattice planes can be clearlyseen. From the integrated profile of the area in FIG. 7A the sharpnessof the interfaces was determined to be 1-3 lattice spacings. The averagespacing between the lattice fringes in the lighter band is 0.344 nm,corresponding well to d111=0.338 nm of InP. FIG. 7B is aone-dimensionally integrated profile of the boxed area in A. The widthof the barrier is about 5.5 nm (16 lattice spacings), and the interfacesharpness is of the order of 1-3 lattice spacings, judged by the jump inimage contrast. The background is not linear due to bend and straincontrast around the interfaces. The difference in lattice spacingbetween the InP and the Inks is 3.4%, which corresponds well with thetheoretical value of the lattice mismatch (3.3%).

Since the heterointerfaces were determined to be abrupt enough formaking high quality quantum devices, double-barrier resonant tunnelingdevices may therefore be envisaged. A barrier thickness of about 5 nmwas chosen. In FIG. 8A a TEM image of such a double barrier devicestructure formed inside a 40 nm wide nanowhisker can be seen. Thebarrier thickness is roughly 5 nm on either side of the 15 nm thick InAsquantum dot. Below the TEM image (FIG. 8B) the energy band diagramexpected for the device is shown, with the longitudinal confinement(z-direction) determined by the length of the dot and the lateralconfinement (perpendicular direction) depending on the diameter of thewhisker. For this device only the lowest transverse quantized level wasoccupied (splitting of the order of 5 meV), with the Fermi energyindicated, determining the highest occupied longitudinal states filledwith electrons. In between the two InP barriers the fully quantizedlevels of the central quantum dot are indicated, with the same sequenceas schematically indicated in the emitter region for the transversequantized levels, but with a greater splitting (of the order of 100 meV)between the longitudinal quantized states in the quantum dot and anapproximate quantization energy for the ground state of E1z=40 meV. Atzero applied bias, the current should be zero since no electronic statesin the emitter are aligned with any states in the central dot because ofthe difference in energy quantization between the dot and the emitter.As the bias is increased the states in the dot will move towards lowerenergy and, as soon as the lowest dot-state is aligned with the Fermilevel, the current starts to increase (here the Fermi level is assumedto lie between the two lowest states in the emitter). When the dot-statefalls below the energy level of the first emitter state the currentagain drops to zero, resulting in the characteristic negativedifferential resistance.

The electrical properties of this 1D DBRT device are presented in FIG.8C, showing almost ideal I-V characteristics, as expected for such adevice. The I-V trace shows no current below a bias of around 70 mV,corresponding to the bias condition for which electrons must penetrateboth barriers plus the central InAs segment to move from the emitter tothe collector. At a bias of about 80 mV a sharp peak is seen in the I-Vcharacteristics, with a half-width of about 5 my in bias (which can betranslated into an energy sharpness of the resonance of about 1-2 meV).The peak-to-valley ratio of the 80 mV peak is extremely high, about50:1, and was seen in different samples investigated. After the deepvalley, the current increases again for a bias of about 100 mV, withsome unresolved shoulder features observed on the rising slope. Notethat the I-V trace for increasing bias voltage coincides with that fordecreasing bias voltage indicating that the device characteristics arehighly reproducible and exhibit negligible hysteresis effects. Inaddition, the 80 mV appears similarly in the reverse bias polarity. Inthis case the peak is only slightly shifted (5 mV) suggesting a highsymmetry of the device structure. Accordingly, these results report theinvestigation of the materials and barrier properties of singleheterostructure barriers inside semiconductor nanowires, bridging thegap from thick barriers, for which only thermal excitation above thebarrier is possible, down to single barrier thickness, for whichtunneling through the barrier dominates.

With this approach one-dimensional, double-barrier resonant tunnelingdevices have been prepared, with high-quality device properties, and anenergy sharpness of about 1 meV and peak-to-valley current ratio of50:1.

Referring now to FIG. 9, a preferred embodiment of a resonant tunnelingdiode is shown, having a nanowhisker 40 extending between collector andemitter contacts 42, 44, 2 microns apart. First and second InAs portions46, 48 of the whisker make electrical contact with respective contacts42, 44. Barrier portions 50,52 of InP separate a central quantum dot orquantum well portion of InAs, 54, from the emitter and collectorportions. The length of the portion 54 is around 30 nm. The precisedimensions will be selected in dependence upon bandgap barrier height,etc., in order to achieve appropriate quantum confinement.

The diode operates in the conventional way of RTDs; for an explanationof the theory of operation; see, for example, Ferry and Goldnick,Transport in Nanostructures, CUP 1999, pp 94 et seq.

In the RTD of FIG. 9, the segments 50, 52 may be replaced by a wide bandgap insulating material, in the manner shown in FIG. 10. Referring toFIG. 10, an embodiment is shown having an insulating segment. Agermanium whisker 100 is grown by the processes described above, havinga short segment 102 of silicon. Lattice mismatch is accommodated byradial outward expansion of the whisker. This silicon dot is oxidised byheat to give a large silicon dioxide spacer 104 within the germaniumwhisker. This has an extremely stable large bandgap offset. Aluminiumcan be used instead of silicon. This embodiment can be used for examplefor tunneling effects, in the embodiment of FIG. 9.

As regards making electrical contacts with the collector and emitterportions of the embodiment of FIG. 9, this can be done in differentways. The whisker may be positioned across large metallised bond pads,as shown in FIG. 9. Alternatively, the nanowhisker may be positioned ona substrate, its position identified by a suitable scanning method, andthen bond pads may be formed over the ends of the whisker by ametallization process. Another alternative is to leave the nanowhiskerextending from the substrate, where it makes contact at its base with anelectrical contact, to encapsulate the whisker in a resin or glassysubstance, and then form an electrode over the encapsulation, makingelectrical contact with the whisker tip. This latter method may be moresuitable for integration with other electrical components and circuits.

Referring now to FIGS. 11 to 14, an embodiment of the invention isdisclosed which comprises a heterojunction bipolar transistor(heterobipolar transistor; HBT); this differs from the conventionalbipolar transistor in that different band gap materials are used in thetransistor. For example, a nanowhisker 110 may have an emitter segment112 of GaP, connected to a base segment 114 of p-doped Si, which is inturn connected to an n-doped collector segment 116 of Si. Metallisationelectrodes 118 make contact with the respective segments 112, 114, and116. FIG. 12 shows a band gap diagram for the HBT. By reason of therelatively wide band gap of the emitter, minority current flow from thebase to the emitter is inhibited. The depletion area between the baseand collector is characterized by a gradual change in doping from p-typeto n-type. As an alternative, the base and collector may be formed ofternary or quaternary materials, being a stoichiometric composition, andthe composition gradually changes over a large number of lattice planes,say 100 to 1000, to give the required depletion region field. Change inenergy band gap with composition is shown in FIG. 13 for the ternarymixture Al_(x)Ga_(1-x)As.

FIG. 14 shows variation in bandgap energy and lattice parameters for avariety of III-V materials. It will be appreciated that with the methodof forming nanowhiskers according to the invention, it is possible toform heteroepitaxial junctions of materials with widely differentlattice parameters, e.g. GaN/AlP, the lattice mismatch beingaccommodated by radial bulging of the whisker.

Photonics Components

Referring to FIG. 15, this shows schematically an extremely small LEDcapable of single photon emission. Single photon emission is ofimportance, for example for quantum photography or detection ofindividual molecules of molecular species. A whisker 150 has anode andcathode outer regions 152 of indium phosphide either side of an innerregion 156 formed of indium arsenide, so as to define a quantum well.Regions 152 are connected to respective anode and cathode electricalcontacts, formed as metallisation areas 158. In contrast to planardevices, where because of the need for lattice matching and forrelieving mismatch strain, only certain wavelengths are possible, animportant point of this embodiment is that the wavelength of the LED isfully variable since the materials making up the diode may be of anydesired composition to achieve a desired wavelength of emission (seeFIG. 14 discussed above), since lattice mismatch is accommodate byradial outward bulging of the whisker. Since the materials may bestoichiometric compositions, the wavelength is continuously variableacross the range from 1.5 ev to 0.35 ev. A one-dimensional structurerequires much less processing than prior art layered structures and ismade by a self-assembly process, with the whole structure between theelectrical contacts. If a laser construction is required, Fabry Perot(FP) cleavage planes 159 are formed spaced an appropriate distanceapart. As an alternative, regions 159 are formed as mirrors comprisingsuperlattices. The superlattices may be formed as alternating sequencesof InP/InAs, the sequence alternating over segments of only a fewlattice planes, as is known to those skilled in the art.

LEDs, lasers, and other micro cavity structures are often fabricatedwith gallium nitride (GaN). Whilst nitrides have a number of advantages,particularly in optics, problems with nitrides are that firstly they arefilled with dislocations and that secondly there is a lack of suitablesubstrates (sapphire being one commonly used substrate). Whiskers can bemade with defect-free nitrides, and there is not a problem of latticematching to a substrate. A regular FP laser can be made, with thestructure of FIG. 15, with dimensions less than 300 nm, preferably ofthe order of 100 nm. It is a bottom up structure, which is well suitedto reading DVDs and writing thereto. Nitride systems are quite wellsuited for whisker growth.

The light source-emitting region 156 can be made as small as about 20nm³. This represents an extreme example of a point source and can beused, as indicated schematically in FIG. 16 to locally excite individualbiological cells 160. The light source 156 provides a near field 162(exponentially decaying) which excites the cell 160 since the physicalspacing between the light source and object is a fraction of awavelength. It is of use in DNA sequencing, and, as shown, the source156 may be mounted in a groove 164 of a glass capillary tube 166. Thecell flows along the tube as part of a fluid mixture, and flows past thesource 156.

Referring to FIG. 17, this shows an embodiment of the invention adaptedfor Nano Imprint Lithography (NIL), where an array 170 of whiskers 156,providing point sources of light, are individually addressable by anenergisation source 172. The array is mounted on a carriage 174 movableover the surface of a resist material 176. The carriage is movable insteps of 20 nm, and at each step, the whiskers 156 are selectivelyenergised in order to illuminate the material 176 with near field light,and to create a desired developable pattern in the resist 176.

Referring to FIG. 18A, a photodetector is shown in accordance with theinvention. For example, a nanowhisker 180 may extend between metallisedcontact pads 182. There is typically a high contact resistance, between10 KO to 100 KO, arising from small contact areas between pads 182, andwhisker 180. The whisker may comprise an n-doped indium phosphideportion 184, and a p-doped indium phosphide portion 186, with a p-njunction 188 between, which may be abrupt, or may extend over a largenumber of lattice planes. This arrangement is suitable for detectinglight with wavelengths 1.3 micron or 1.55 microns. As indicated in FIG.14, any desired compositional “match” may be used, and therefore thematerials can be modified for detection of any wavelength, from 1.55microns or less. As an alternative, a PIN or Schottky diode structuremay be used. A PIN structure, as shown in FIG. 18B has an intrinsicsemiconductor material segment 188 between the two semiconductorportions 184 and 186. The whisker is constructed as described withreference to FIG. 10. A Schottky diode structure, as shown in FIG. 18Chas a base portion 189 formed as a metallisation contact from which thewhisker extends; the interface between the contact and the whisker formsthe Schottky diode. The lower frequency limit on detection of radiationis in the terahertz region of the electromagnetic spectrum.

Referring to FIG. 19A, a solar cell application is shown for thephotodetector structures of FIG. 18. Millions of whiskers 190, eachhaving p- and n-doped portions 191, 192 are formed on a substrate 193,doped (P+). The whiskers are formed by growth using gold, or other,nanoparticles, deposited onto substrate 193, e.g., from an aerosol. Thewhiskers may be encapsulated in plastics 194 and have a transparent tinoxide electrode 196 on the upper surface, which makes contact with thefree ends of the whiskers to permit electrical current to flow along thelength of the whiskers. The structure is extremely efficient in trappinglight since each whisker is 100% reliable. The overall efficiency isbetween 35 and 50% and is of use in multi-bandgap solar cells. Bycontrast amorphous silicon grown at 300° C. gives an efficiency of about10%. Crystalline silicon gives an efficiency of about 15% and specialpurpose III-V solar cells for space applications are grown at 400° C.and have an efficiency of up to 25%. Grätzel solar cells for spaceapplications have titanium dioxide nanoparticles painted on solarpanels, with an appropriate dye; such cells have an efficiency up toabout 8%.

Referring to the modification shown in FIG. 19B, each whisker of thesolar cell array is modified to the form shown 197, with differentsegments of different materials 198 along its length. These materialsare selected so that the p-n junctions absorb light at differentwavelengths. The point along the whisker at which the whisker is mostsensitive to light of a particular wavelength depends on the precisestructure of the solar cell and factors such as reflection andrefraction within the structure.

The embodiment of FIGS. 19A-B is inexpensive, since the growthconditions are inexpensive, and further only very small quantities ofexpensive materials are required. In alternative constructions, thewhiskers can be silicon (least expensive) or germanium. The length ofthe whiskers is 1 or 2 microns. A PN junction is achieved by doping thewhisker along part of its length, or by forming Schottky barriers, asindicated in FIG. 18C at the base of the whisker.

Referring to FIG. 20, an embodiment is shown, which is a source of verylong wavelength infrared radiation, e.g., at terahertz frequencies. Anindium phosphide nanowhisker 200 has a series of very thin indiumarsenide stripes 202, separated by spacer stripes 204 of indiumphosphide. The stripes are grown by the process described above. Eachstripe 202, 204 is a few lattice planes wide, and the stripes create asuperlattice 206. By applying a voltage across electrode contacts 208,electrons move across the superlattice. The superlattice creates aseries of quantum well bandgaps (potential wells) which, according tothe Bloch theorem will give a conduction band with allowable regions ofelectron wave number or momentum k—these allowable regions correspond toterahertz frequencies, thereby to create terahertz emission.

FIGS. 21A-21D illustrate an embodiment of the invention, implemented asa photonic crystal. Photonic crystals are well known—see for examplecopending application WO 01/77726. In the main, prior methods of formingphotonic crystals involve etching air holes in a substrate according toa predetermined lattice pattern. A concept of this embodiment is to usea patterning technique for defining a crystal lattice pattern on asubstrate, but to grow nanowhiskers to define the crystal, rather thanetching holes. This has numerous advantages in that etching techniquesare not as reliable (etching harms the substrate surface) as a bottom uptechnique of growing whiskers. Therefore the whisker technique is moreaccurate and gives higher quality; and simplicity, as well as economy inthat fewer process steps are required.

Referring to FIG. 21A, a substrate 210 has a triangular lattice patternof square patches 212 of gold about 300 nm², spaced apart by a distanceof 300 nm, the patches having been formed by ebeam lithography, UVlithography or a nanoimprint lithography (NIL) process. The substrate isinitially prepared before gold deposition as a clean substrate withoutoxide contaminants. The substrate is heated to melt the gold rectanglesso that they form balls 214, about 100 nm diameter, as shown in FIG.21B, which are then annealed. Whiskers 216 are then grown by the processas described in Example 1, about 100 nm wide to form a photonicscrystal, as shown in FIG. 21C.

It is possible in accordance with the invention to definethree-dimensional photonic crystals by whisker formation. This can bedone as indicated in FIG. 21D by forming each whisker with a sequence ofsegments 217, 218 of different materials, for example an alternatingsequence of III-V materials such as InAs/GaAs, or group IV materialssuch as Ge/Si, in accordance with the method of Example 2, so that atintervals along each whisker, segments are provided with an appropriaterefractive index to form a photonic band gap.

Single Crystal Layers of III-V Materials

Referring to FIGS. 22A-22G, an embodiment of the invention is shown forgrowing epitaxial layers of a desired material on a substrate. As shownin FIGS. 22A & B, a silicon or gallium arsenide substrate 220 has formedon an upper surface rectangles 222 of gold, indium or gallium, which arepositioned on the substrate by a stamp 223 in a NIL process or asdescribed in Example 1. An epitaxial mask deposit 224 a few nanometerswide of dielectric material, for example, silicon dioxide or siliconnitride, are formed over the substrate 220 and around rectangles 222.Heat is applied to anneal the rectangle to balls 226, FIG. 22C, andwhiskers 228, FIG. 22D, are grown of for example InP or GaAs.Alternatively a carbon-based material is used as the deposit 224 (acarbon based material stabilises the particle when the ball is formed byannealing, the dielectric material being desorbed). The balls are usedas seed openings for bulk growth i.e. a layer of the desired material.The dielectric layer prevents atomic bonding and lattice mismatch effectbetween the substrate and the crystal layer. The whiskers grow togetherwith a bulk layer of InP or GaAs 229, FIG. 22E. There are gradualchanges in growth conditions from the whisker to the layer. Thus thereis nucleation on the whiskers without creating defects. There are smallnucleation steps and strain effects do not appear to give dislocations.Where the substrate is a III-V material, the important advantage is tocreate a lattice-mismatched layer on the substrate without gettingmisfit dislocations.

In a variation, as shown in FIG. 22F, gold balls 226 are deposited onthe surface from an aerosol, in accordance with the method of Example 1.The epitaxial mask deposit 224 is formed over the balls. Whiskers arethen grown, as in FIG. 22D.

In a further development in accordance with the invention, it is knownthat whiskers tend to grow preferentially in the <111>B directionbecause for gallium arsenide (a zinc blende lattice), the arsenic atomis at the apex of a pyramid with gallium ions at the base of thepyramid, see FIG. 23A. A preferred embodiment of the invention isillustrated in FIG. 23B, where a substrate 230 of silicon has a serratedsurface having V-grooves 232 of microscopic dimensions etched to expose<111> planes. Gold particles 234 are deposited on the surfaces of theV-grooves. GaAs whiskers 236, shown in ghost form in FIG. 23C, and grownin accordance with Example 1, will extend perpendicular to the walls ofthe serrations. These whiskers provide nucleation points for bulk growthof a GaAs layer 238. There are gradual changes in growth conditions fromthe whisker to the layer. Thus there is nucleation on gallium arsenidewithout creating defects. Any small nucleation steps and strain effectsdo not appear to give dislocations. The direction of the whiskers, in<111> directions at an angle to the substrate, forces epitaxial growthin a certain direction, and takes away the problem of antiphase domains,which has been a problem. Thus this provides a way of integrating III-Vcompounds onto silicon (or other Group IV) substrates, and is cheaperthan existing methods—see for example PCT Published Patent ApplicationNo. WO 02/01648.

A further advantage of a V-grooved substrate arises in connection withthe solar cell application of FIG. 19, in that the serrated substrateprovides multiple reflections of incident light, and hence an increasedprobability of photon capture.

Referring now to FIG. 24, a preferred embodiment is described forcontrolling the orientation of whiskers. Normally, as described above,whiskers of III-V compounds grow in the <111>B direction. A problem hereis that such whiskers change more or less randomly between hexagonal(wurtzite) (FIG. 24A) and cubic (zinc blende) (FIG. 23A) structures.This gives rise to many stacking faults. Stacking faults are always aproblem particularly for optical properties, but also for electricalcharacteristics. By applying strain to the whisker during formation, bychange of growth conditions, the direction of growth of the whisker canbe changed to the <100> direction, which gives a cubic lattice structure(zinc blende), which does not have stacking faults.

In FIG. 24B, a silicon substrate 240 with a <100> surface has whiskers242 of, e.g., InP, grown on it. The whiskers start to grow as at 244 inthe <111> direction, but shortly after initial growth, operatingconditions are changed, by increasing the rate of growth and increasingthe temperature and pressure within the CBE apparatus, so that thewhisker continues to grow as at 246 in the <100> direction. The point248 at which direction changes is a <110> facet. The whisker at thetransition maintains its epitaxial crystalline nature. The structure ofthe crystal in segment 246 is hexagonal close packed, whichsignificantly reduces the problem of stacking faults.

In an alternative method of growth, a short barrier segment of a wideband gap material, e.g. InAs, is grown at point 248; this has the sameeffect of changing the subsequent orientation of the whisker.

This embodiment is therefore particularly suitable for the growth ofnitrides, e.g. GaN, which preferentially grow as hexagonal lattices, andwhich are particularly prone to stacking faults. By “forcing” thenitride crystal to grow in cubic form, stacking faults are reduced.Further, where structures are made in accordance with Example 2 withsegments of different material along the whisker, micro-cavitystructures for gallium nitride lasers can be developed. Nitride systemsare quite well suited for whisker growth. The problem with nitrides isthat they are filled with dislocations and the lack of suitablesubstrates. Whiskers can be made with defect-free nitrides, and theproblem of lattice matching is not there. A regular FP laser can be madein a nanowhisker less than 300 nm length, of the order of 100 nm. It isa bottom up structure, which is well suited to reading and writing toDVDs.

Referring now to the embodiment shown in FIG. 25, this embodimentrelates to field emission tips or Spindt cathodes. These are of use infield emission displays (FED), and many methods have been proposed formaking such displays. One prior art arrangement as shown in FIG. 25Acomprises a silicon substrate 250, with a surface 252, which ispatterned by laser ablation, or the like, to form microscopic ornanometric tips 253. A phosphor screen 254 is positioned adjacent thetips, and a voltage between the tips and the screen generates' extremelyhigh field strengths at the tips, which causes current flow into thescreen, and thus radiation of visible light from the screen.

In FIG. 25B, an embodiment of the invention is shown, comprising an FED,wherein the elements of the display are individually addressable. Etchedcontact metallisation areas 256 are formed on a silicon substrate 250.Gold seed particles 258 are positioned on each metallisation area, bythe method as described in Example 1. The gold particles are used asseeds for whisker growth, in order to grow Si whiskers 259, each whiskerextending from a respective metallisation area. A single whisker, asshown, or a group of nanowhiskers, forming a single display element mayextend from a respective metallisation area. In addition to beingindividually addressable, an advantage of this embodiment is that theFED is 100% reliable, in contrast to prior methods, e.g. carbonnanotubes (CNT).

FIG. 26 discloses an embodiment for infrared to visible lightup-conversion. An image 260 of infrared radiation with a wavelength of1.55 or 2.5 μm is shone on the base surface of a gallium arsenidesubstrate 262—a relatively large band gap material which will notinteract with the radiation. The other side of the substrate has indiumarsenide projecting whiskers 264, grown as described in Example 1, andhaving a relatively small band gap, which will cause absorption of thephotons of the radiation. Whiskers 264 are not however individuallyaddressable, in contrast to FIG. 25. A voltage of about 20-50 volts isapplied between the ends of the whiskers and a nearby fluorescent screen266, and electrons are generated from the indium arsenide whiskers.Indium arsenide has a bandgap corresponding to 3 microns, and willtherefore produce electrons in response to radiation shorter than 3microns. Gallium phosphide may be used as an alternative, but this has avisible light bandgap. The emitted electrons cause fluorescence to givevisible light 268 emitted from the fluorescent screen, and a version ofthe image, but up-converted to visible light wavelength. The appliedvoltage may be raised sufficiently to induce avalanche effects.

FIG. 27 discloses an embodiment of the invention in which a whisker 270,400 nm long of GaAs (made in accordance with Example 1) extends from ametallisation contact area 272 on a silicon substrate 274. Thisdimension is ¼ of a wavelength of 1.55 micron radiation, and hence thewhisker provides a ?/4 resonant antenna for 1.55 micron radiation.Contact area 272 provides a ground plane. The antenna may be positionedto receive radiation 276 in free space; alternatively, it may bepositioned adjacent the end of a silica fibre link 278 for detection ofradiation in the third optical window.

Referring now to FIG. 28, an embodiment of the invention is shown foruse in the field of spintronics. Spintronics is a technical field wherethe properties of electronic devices rely on the transport of electronspin through the device—see for example Scientific American June 2002 pp52-59, “Spintronics”, David D. Awschalom et al. In FIG. 28, a whisker280, formed by the process of Example 1, of a magnetic or semi-magneticmaterial such as manganese gallium arsenide (semi-magnetic) or manganesearsenide (ferromagnetic) is formed on a Si substrate 281. Under anapplied voltage V, spin polarised electrons 283 are emitted from the tipof the whisker, which makes electrical contact with an electricalcontact 284 disposed on a substrate 286. The spin polarised electrons283 are used for reading and writing magnetic storage devices 288disposed on substrate 286.

In a further development of this embodiment, a problem is overcome,which is that, with ferromagnetism, there is normally a lower limit onferromagnetic domain width, about 10-15 nm, below which theferromagnetism changes to super-paramagnetism. However when incorporatedin a nanowhisker, in accordance with the method of Example 1, the domaindiameter can be reduced, because of the reduced possibilities forsymmetrical alignment in a 1-dimensional system, which makes it moredifficult for the ions of the material to have more than oneorientation. The material of the whisker can be iron, cobalt, manganese,or an alloy thereof.

Referring now to FIG. 29, a further embodiment of the invention is showncomprising a substrate with an array of electrodes for implantation intoa nerve for repairing a nerve function, for example the retina of aneye. The electrodes are individually addressable. Etched contactmetallisation areas 350 are formed on silicon substrate 352. Gold seedparticles 354 are positioned on each metallisation area, by the methodas described above. The gold particles are used as seeds for whiskergrowth, in order to grow silicon whiskers 358, each whisker extendingfrom a respective metallisation area. A single whisker, as shown, or agroup of nanowhiskers, forming a single electrode element may extendfrom a respective metallisation area. In addition to being individuallyaddressable, an advantage of this embodiment is that the electrodes are100% reliable.

Referring now to FIG. 30, a further embodiment is shown comprising ananowhisker 360 formed by the method described above. The whisker isformed of silicon and has a gold particle melt at one end 362.Subsequent to formation of the whisker, the whisker is exposed to anatmosphere at a suitable temperature for oxidation of the silicon. Thisforms an outer shell 364 of silicon dioxide surrounding the whisker andextending along its length. The gold particle melt 362 remains in anunoxidised condition. This therefore provides a structure highlysuitable for the electrode assembly shown in FIG. 29, wherein theelectrode has very precise electrical characteristics. The siliconmaterial may be replaced by any other material that can be oxidised.

As an alternative, the whisker 360 may be exposed to an atmosphere of asuitable material for forming a high band gap material as an alternativeto the oxidation layer 364.

Referring now to FIG. 31, this shows a further embodiment of theinvention comprising a silicon base member 370. This base member may bea planar substrate, or just a bar. In any event, a row of nanowhiskers372 is formed from one edge surface of the bar or substrate. Thenanowhiskers are regularly spaced apart and project into space. Thenanowhiskers may have a coating formed on them for absorbing certainmolecular structures. In any event the cantilever beam arrangement maybe used for any of the well-known applications for cantileverarrangements for measuring molecular species etc.

Referring to FIG. 32 this shows a further embodiment of the inventioncomprising a molecular sensing device. A substrate 380, e.g., of siliconnitride, has an insulating layer 382 formed thereon, with a conductivesurface 384, for example gold. An aperture 386 is formed within thelayers 382, 384 and a nanowhisker 388 is formed within the aperture.

This is done essentially by a self-assembly process, since the apertureis formed in insulating layer 382 and the gold layer 384 is subsequentlydeposited. Gold is therefore in consequence deposited on the base of theaperture, indicated at 389, and upon heating forms a gold particle meltwhich enables formation of a nanowhisker with appropriate conditions.The gold particle melt 389 resides on top of the nanowhisker in thefinished nanowhisker. The nanowhisker height is such that the particlemelt 389 is at least approximately co-planar with the gold surface layer384.

The natural resilience of the nanowhisker implies that it has acharacteristic frequency of vibration from side to side in a directiontransverse to its length. Oscillation of particle melt 389 can bedetected by voltage or current signals being created in conductive layer384. This therefore provides a means of detecting the frequency ofvibration of the nanowhisker 388.

By appropriate activation of the conductive material with an appliedvoltage, the whisker may be made to mechanically vibrate within theaperture at a certain eigen frequency, for example, in the gigahertzrange. This is because, in view of the small dimensions and low currentsinvolved, during the period of a single vibration, a single electron istransferred from one side of the conductive material to the other viathe seed particle melt. This creates a current standard generator, wherethe current I through the conductive material is equal to product of thefrequency of vibration f and the charge e of an electron: I=f·e. Thus aknown reference signal is generated which can be used in appropriatecircumstances.

In addition, the particle melt 389 may be coated with a receptorsubstance so as to permit certain molecular species to be absorbed onthe surface of the particle melt 389. This will cause a change incharacteristic frequency of the nanowhisker. This change in frequencymay be detected and provides a means of computing the weight of themolecular species absorbed on the surface of the melt 389.

FIG. 33 shows the tip of a Scanning Tunneling Microscope (STM) ascomprising a nanowhisker 392 of InP formed on the end of a flexible beam394 of Silicon. Beam 394 is formed by etching from a substrate or bar.

The invention claimed is:
 1. A photonic crystal, comprising a substrate,and an array of one-dimensional nanoelements extending from one side ofthe substrate, each element extending upright from the substrate, andhaving a substantially constant diameter of nanometer dimension, whereinthe array of nanoelements is arranged in a two-dimensional lattice,whereby to provide a photonic band gap for incident electromagneticradiation; and wherein the nanoelements are spaced apart by a distanceof about 300 nm.
 2. A photonic crystal according to claim 1, wherein thediameter of each nanoelement is not greater than about 100 nm.
 3. Aphotonic crystal according to claim 1, wherein each nanoelementcomprises a nanowhisker having a plurality of lengthwise segments of afirst type, comprised of a material having a first refractive index andhaving a first predetermined length, said segments of said first typealternating with at least one segment of a second type, comprised of amaterial having a second refractive index and having a secondpredetermined length, said first and second refractive indices and saidfirst and second predetermined lengths being selected to form a threedimensional photonic crystal.
 4. A photonic crystal according to claim1, wherein each nanoelement comprises a semiconductor light emittingnanowhisker.